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Author Topic: HELP!!! High SWR and a hamstick  (Read 985 times)
KE5ATE
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Posts: 8




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« on: February 04, 2008, 06:49:46 PM »

I'm new to hf radio. I've got an FT-897D and a tram 2mtr/440 antenna that works great (swr 1). I just got a workman 20 meter hamstick, I can not get the antenna to tune at all, the swr is 3+ no matter where I put the antenna, I tried putting it off of the right rear of the bumper and figured that maybe it was because there was no ground plane and tried moving it to a homemade mount screwed to the center front of the trunk lid and it is still the same, any ideas? bad antenna maybe?

KE5ATE
Allen
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K0BG
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« Reply #1 on: February 05, 2008, 05:38:30 AM »

Yes, visit my web site.

First of all, a low SWR has nothing to do with efficiency or suitability. It just means the input impedance where the coax connects to what ever device is measuring the SWR, is close to line Z.

One of the problems with the Hamsticks, and similar antennas, is the whip can slide down into the coil portion (the mast is hollow). This fact makes tuning difficult at best. While you can use an SWR bridge to adjust the resonant point, an antenna analyzer makes the job much simpler.

The resonant point, even for a 20 meter Hamstick is rather touchy. As little as 1/4 inch movement might be too much.

Alan, KØBG
www.k0bg.com
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KE5ATE
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« Reply #2 on: February 05, 2008, 06:46:16 AM »

Hi Alan, thanks, I've tried moving the whip from all the way out to all the way in a half inch at a time and there was never any change at all in the SWR, I don't have access to a antenna analyzer.

KE5ATE
Allen
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K0BG
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« Reply #3 on: February 05, 2008, 09:04:38 AM »

1). Make sure there is continuity between the coax shield and the trunk lid. If there isn't, fix it.

2). Strap the trunk lid across both hinges. The trunk lid by itself isn't much a a image plane.

3). After you do steps 1 and 2, set the whip length EXACTLY where they tell you to set it. Check the CW portion, in the middle, and then the top of the phone band. If it is higher on the low end the whip is too short. If the SWR is higher on the top end, it's too long.

4). Move the whip about 1/4 inch at a time; 1/2 inch is too much.

5). If you get the same SWR readings on all portions, then use a decent SWR bridge rather than the built in one. They're suspect to begin with. Also, check the continuity of the coax itself, especially if you're the one that attached the connectors.

6). If this doesn't work, then find someone with an analyzer to help you.

Alan, KØBG
www.k0bg.com
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KE5ATE
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« Reply #4 on: February 08, 2008, 08:11:42 AM »

Antenna was bad, exchanged it and got one that actually is tuneable.

KE5ATE
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NA0AA
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« Reply #5 on: February 08, 2008, 06:34:49 PM »

I just installed one today - the whip is way too long from the factory, I had to cut off about 10" of it in order to keep the end out of the coil [see the caution about the wire inside the hollow portion].  Ended up with 31.5" of whip extended.  Match is under 1.5 in the entire phone portion of 20.

I did provide a counterpoise wire attached to the roof rack of the car.  This greatly improved the match.  I sure hope the roof rack mount is strong enough to hold this antenna.

Look forward to seeing how it talks.
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AB7KT
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« Reply #6 on: February 09, 2008, 08:09:31 PM »

When installing an antenna, you first need to determine if it is too long or too short.
This is done by going to the band you are using and dialing the radio to the lowest frequency you are allowed to use on that band and seeing what the SWR is. Then go to the highest frequncy you are allowed to use on that band and see what the SWR is there. One will be lower than the other. If the lower frequency has the lower SWR, the antenna is long. If the higher frequency has the lower SWR, The antenna is short.
This is a logical approach that uses the theory we learn as hams rather than a random, shotgun approach.
It was mentioned in the last post that these whips are often long. I have found that to be true most of the time. You push the whip all the way into the coil and the resonent frequency is still lower than we want it. That means we have to cut the whip shorter. This is done by taking a file and filing a deep notch into the whip and then snapping the end off, at the notch with a pair of pliers.
I bet that is what happened with your first antenna.
How was it determined that the antenna was "bad" ?



















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I thought you said this was a weak signal mode ? I HAVE a weak signal and he still didn't hear me.

FWIW: My callsign is AB8KT
KE5ATE
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Posts: 8




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« Reply #7 on: February 09, 2008, 08:57:57 PM »

The antenna swr was maxed out from 14,075 - 14.350. it just maxed the meter out no matter how long the whip was it never changed. Exchanged the antenna, took the whip off of the bad one and screwed it onto the new one and the swr was already at 2 on 14.280, the antenna is now tuned to 1.2 swr on 14.075- 14.350. works great. had to cut 1 inch off of it.

Thanks for all the help
KE5ATE
Allen
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KL7AJ
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« Reply #8 on: February 15, 2008, 12:51:06 PM »

Loaded H.F. mobile antennas have very high Q, and the SWR will rise faster due to reactance than due to impedance mismatch.  This is where a grid dip oscillator is SO helpful.  See if you can find out where your antenna/loading coil combination is resonant with a GDO.  (Beg borrow or steal one of these, if you don't have one).  If it IS resonant in the band, you might not get much lower than a 3:1 SWR, without a matching section.  But you definitely WON'T get a low SWR if it ISN'T resonant.

The rule for mobile antennas is RESONATE first, MATCH second.


eric
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K7AAT
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Posts: 414




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« Reply #9 on: February 19, 2008, 07:27:38 PM »


   "Antenna was bad, exchanged it and got one that actually is tuneable." nKE5ATE

    Glad you got it replaced.  I have had more than one ham stick go bad... due to the coil wire either fractures and/or comes unsoldered at the bottom of the stick were it is soldered to the metal 3/8x24 threaded chrome metal piece.  These are repairable, with a LOT of heat,  but getting a replacement is better.

    73

    Ed   K7AAT

 
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